In my case, I stumbled across it quite by accident. I think many states have regional Special Needs Advocates, and just with a Google search, I found:

http://www.education-a-must.com/advocates.html

Gifted advocates are rarer. The one I used was PA-based and cost money, though he wasn't unreasonable about it. I also found that I only really needed the help for about a year and a half. After that, I was much more confident that I knew the process.

This Hoagies page has regional organizations that could probably point you at good local advocates:

http://www.hoagiesgifted.com/organizations.htm#usa